1. Field of the Invention
The present invention relates to a filter circuit suitable for extraction of a signal of the desired frequency band and particularly to a filter circuit which is useful for selection of a particular channel signal in a mobile radio system.
2. Description of the Related Art
In a hand-held transceiver, for example, a mobile radio or a portable type telephone set, etc. which is operated as a terminal of mobile communication system, an audio data is modulated by many sub-carriers frequency-divided for the high frequency (GHz) carrier frequency assigned to such radio system and moreover modulated to the channel of a particular frequency band in which each sub-carrier frequency is divided into the time slots of several tens of channels for the purpose of transmission and reception thereof.
In such a mobile radio system, an input receiving signal level received by a terminal changes to a large extent depending on the operating environment and it must be assumed that such input receiving signal level changes as much as 80 dB or more, for example, depending on distance between the terminal and base station and level changes caused by fading.
Moreover, it can also be thought that such a bad environment that the signal level of the target channel is low and there is an intensive interference wave for the adjacent channels exists.
Therefore, a high frequency circuit of the double-super receiving system is generally employed in the transmitting and receiving system of such mobile radio but as a filter circuit to accurately extract the target frequency, a filter using a bulk element (saw filter, piezoelectric element, etc.) is mainly used to cover the high sharpness and wide dynamic range.
However, since the hand-held terminal is required to be small in size and light weight, it is considered, for example, to use an active filter formed of an IC circuit as a filter circuit provided between intermediate frequency stages, etc.
FIG. 1 shows an example of the receiving high frequency circuit of a terminal.
The radio wave transmitted from the base station is received by an antenna 100, it is then amplified by a high frequency amplifier 101 consisting of a low noise GaAs FET element, etc., for example, and this signal is converted to a first intermediate frequency through the mixing with a signal of a first local oscillator 103 being supplied to a first mixer 102. The predetermined frequency band of this converted first intermediate frequency is extracted by a saw filter 104 and it is supplied to a second mixer 106 after it is amplified by an intermediate amplifier 105.
A signal is also supplied to the second mixer circuit 106 from a second local oscillator 107 for selection of the desired channel and thereby the signal of the frequency band of the target channel can be obtained as the second intermediate frequency.
Therefore, an active filter circuit 109 is provided to select the signal of the second intermediate frequency band. However, since the active filter circuit 109 for selecting the channel of the target frequency band has a lower dynamic range, it is required, as explained above, to provide a gain varying amplifier 108 in the preceding stage of this active filter circuit 109 and the gain of the gain varying amplifier 108 is controlled by the signal fed back from the level detecting circuit 110 for detecting output signal level of the active filter.
The signal of the target frequency band extracted by the active filter circuit 109 is waveform-shaped to have a constant amplitude by a multi-stage connected limiter circuit 111 and is then input to a signal processing circuit 120 for extracting data.
However, for example, an active filter circuit 109 consisting of an inductance element, etc. formed by combining a resistance element and a capacitance element or active element (transistor, amplifier) is effective for reduction in size and weight of the hand-held terminal and also for reduction in power but in the IC circuit, a gm-C circuit utilizing transconductance of a pair of differential transistors is often used to realize a filter having the high precision frequency characteristic. Therefore, such filter circuit is generally limited to operate in the stable filter characteristic only for the range in which the amplifying element operates linearly.
In addition, when the active filter is designed to have higher sharpness and to assure lower power consumption by the low voltage drive, the following problems are generated.
The first problem is a dynamic range of the feedback loop by the feedback control.
Since the active filter circuit 109 has the frequency characteristic allowing the predetermined frequency to pass by forming inside the complicated feedback loop, this filter can generally be considered as a linear element and therefore the feedback control itself generates a problem when this circuit is provided within the loop of the feedback circuit.
For example, when non-linear behavior of the active filter circuit 109 does not increase monotonously, polarity of the feedback signal is inverted and normal feedback operation is not properly maintained.
In this case, here rise the problems, for example, that intensive signal is momentarily input and after the filter circuit 109 is deeply saturated, the intrinsic level control operation does not start.
Next, the second problem is effective use of dynamic range for the receiving signal.
As shown in FIG. 1, when the gain control is conducted with a signal used to detect an output signal level of the active filter circuit 109 by arranging the gain varying amplifier 108 to the preceding stage of the active filter circuit 109, here rise the problems that the control level La of the desired wave of the frequency band passing through the active filter circuit 109 is different from the control level Lb of the interference wave of the frequency band attenuated by the filter circuit as shown in FIG. 2 and this level differences is uniquely determined by the attenuation characteristic of the active filter circuit 109.
In general, due to characteristic difference of each node voltage forming the active filter circuit 109, the upper limit of receiving range a of the desired wave by saturation of the desired wave itself in the passing frequency band must be set to the level different from that of the upper limit of the mixing range b of the interference wave restricted by masking the desired wave with the saturated interference wave within the rejection band. Moreover, in order to effectively use the dynamic range of the filter, the control level La of the desired wave is preferably set to the upper limit of the receiving range a of the desired wave, while the control level Lb of the interference wave is preferably set based on the upper limit of the mixing range b of the interference wave.
However, if a level difference between the control level La of the desired wave and the control level Lb of the interference wave is uniquely determined only by the attenuation characteristic of the active filter circuit 109, here rises a problem that if it is attempted to control only one level desired most effectively, the other level is not always controlled to the optimum level and the control level for effectively utilizing the operation range of the filter cannot be set.
In order to solve the problems explained above, the filter circuit of the present invention is provided with:
filter means having different gain characteristics for the first frequency band and second frequency band;
gain control means for controlling an input signal including the signals of at least first and second frequency bands and supplying the level-controlled output signal to the filter means;
first level detecting means for detecting the level of output signal of the filter means;
second level detecting means for detecting the input level of the filter means; and
switching means for outputting the detecting signal of higher level of the detecting signals of the first or second level detecting means.
The gain of the gain control means is controlled by the signal output from this switching means.
The filter means may be structured by the controllably connected first and second filter circuits. In this case, control is conducted by detecting an output level of the first filter circuit with the first level detecting means.
Moreover, it is also possible to conduct gain control of the desired wave by supplying an output of the gain control means to the first filter circuit having the desired frequency characteristic and the second filter circuit having the characteristic similar to that of the first filter circuit and then detecting an output signal of the second filter circuit with the first detecting means.
Since the filter circuit of the present invention is designed, as explained above, to control the level of the signal including interference wave to the predetermined value with the second level detecting means to detect the signal to be input to the active filter circuit and to set the control level for the desired wave with the first level detecting means to detect the signal level of the desired wave output from the active filter circuit, the stable level control can be realized even when the filter circuit has the non-linear characteristic and the dynamic range can be used effectively.